According to the Federal Election Commission's filings through June, donors who gave at least $200 to Jeb Bush, Gov. John Kasich, Gov. Chris Christie, or Sen. Lindsey Graham in the Republican primaries have all largely defected from the party. Normally, after a primary is over, the donations shift to the party's nominee. But instead of giving to Trump, they are now supporting Democratic nominee Hillary Clinton.

These donations have been adding up. So far Clinton has received $2.2 million from these Republican party defectors. That's about about $600,000 more than Trump.

That's not great news for the GOP nominee, who filings reveal has received money from a smaller proportion of his party's donors than any candidate since 1980.

This also comes on the back of several high-profile Republican figures coming out in support of Trump's Democratic rival. On Monday, 50 GOP national security experts published an open letter denouncing Trump's candidacy and calling him "a risk to short- and long-term national security."